Technical Abstract:
The main objective was to evaluate methodologies to estimate N losses from stored dairy manure. Manure with high N (HN) and low N (LN) content was obtained from two groups of cows assigned diets of 17 and 15% CP (DM), respectively. Manure collected from the barn floor was diluted with water to 10% DM, loaded in barrels with or without addition of straw, and stored in a partially temperature-controlled environment. Manure samples were collected on days 0 and 136 of storage and analyzed for N, phosphorus (P), and 15N natural abundance (15N). Nitrogen loss was estimated by mass balance and changes in N:P ratio. The study was conducted once in May and twice in October. There were no interactions for the reported measurements. Addition of straw had no effect on N loss measured by mass balance or N:P ratio. Nitrogen losses estimated by mass balance were reduced by 36% on LN relative to HN. Using the N:P ratio, there was a trend for a reduction in N loss for LN relative to HN manure; however, N loss measured using this technique was on average 44% lower than estimated by mass balance. Average 15N abundance in manure increased during storage from 4.965% to 5.995‰, probably due to volatilization of ammonia depleted in 15N, but there was no difference in the magnitude of the increase between the LN and HN manures. Abundance in 15N at day 136 was lower when straw was added (5.603% vs. 6.398‰, P < 0.01), indicating that straw-containing manure may have lost less ammonia N. Results from this study also suggested that N loss was influenced by initial manure N content. Estimates of N losses were substantially lower using the change in N:P ratio, compared with the values observed by mass balance.